The invention relates to a planar transformer comprising a magnetic core as well as a number of layers on which the spiral-shaped winding portions of a primary and secondary coil are provided, whereby winding portions belonging either to a winding of the primary coil or to a winding of the secondary coil are interconnected by means of one or more vias.
A transformer of this type is known from U.S. Pat. No. 5,010,314.
Transformers are necessary in many types of electrical apparatus. In an apparatus which is connected to a mains voltage which is higher than the voltage used at least in parts of the equipment, in general a transformer is used to reduce this voltage.
In general, a transformer comprises a primary coil, a secondary coil and a core. The coils may be made, for example, of copper wire. They may be arranged so as to be juxtaposed. Alternatively, they may be arranged so that one coil surrounds another coil. A coil has one or more windings. As a result of the ongoing reduction in size of electrical apparatus, also the transformers manufactured comprise coils having smaller dimensions. Said coils may be made, for example, of a number of layers of an insulating material on which winding portions of the coils are provided. A transformer of this type is referred to as a multilayer or planar transformer.
The winding portions of a planar transformer may be provided, for example, by means of a printing process. The winding portions of a coil may be externally interconnected. But preferably they are interconnected by means of so called vias. Vias are metallized through holes. If use is made of vias, insulated bridges can be dispensed with, as a result of which the transformer is easier and cheaper to manufacture.
The core of a transformer is preferably made of a material which is a good conductor of magnetic lines of force (for example ferrite). This core is situated partly inside the coils and partly outside the coils. If a current is sent through the primary coil, magnetic flux causes a current to be generated in the secondary coil. The core conducts this flux since it is made of a material having good magneto-conductive properties. During operation, the primary coil is connected to the mains and the secondary coil is connected to the current circuit of the apparatus receiving energy from the mains.
As a result of the ongoing reduction in size of equipment, a further reduction in size of the planar transformers is desirable. A problem associated with a further miniaturization is the higher risk of breakdown during operation, which presents a danger to the user of the equipment.